Diagnostic techniques that allow the practicing clinician to obtain high fidelity views of the anatomical structure of a human body have proved helpful to both the patient and the doctor. Imaging systems providing cross-sectional views such as computed tomographic (CT) x-ray imagers or nuclear magnetic resonance (NMR) machines have provided the ability to improve visualization of the anatomical structure of the human body without surgery or other invasive techniques. The patient can be subjected to scanning techniques of such imaging systems, and the patient's anatomical structure can be reproduced in a form for evaluation by a trained doctor.
The doctor sufficiently experienced in these techniques can evaluate the images of the patient's anatomy and determine if there are any abnormalities present. An abnormality in the form of a tumor appears on the image as a shape that has a discernable contrast with the surrounding area. The difference in contrast is due to the tumor having different imaging properties than the surrounding body tissue. Moreover, the contrasting shape that represents the tumor appears at a location on the image where such a shape would not normally appear with regard to a similar image of a healthy person.
Once a tumor has been identified, several methods of treatment are utilized to remove or destroy the tumor including chemotherapy, radiation therapy and surgery. When chemotherapy is chosen drugs are introduced into the patient's body to destroy the tumor. During the course of treatment, imagers are commonly used to follow the progress of treatment by subjecting the patient to periodic scans and comparing the images taken over the course of the treatment to ascertain any changes in the tumor configurations.
In radiation therapy, the images of the tumor generated by the imager are used by a radiologist to adjust the irradiating device and to direct radiation solely at the tumor while minimizing or eliminating adverse effects to surrounding healthy tissue. During the course of the radiation treatment, the imaging system is also used to follow the progress of the patient in the same manner described above with respect to chemotherapy.
When surgery is used to remove a tumor, the images of the tumor in the patient can guide the surgeon during the operation. By reviewing the images prior to surgery, the surgeon can decide the best strategy for reaching and excising the tumor. After surgery has been performed, further scanning is utilized to evaluate the success of the surgery and the subsequent progress of the patient.
A problem associated with the scanning techniques mentioned above is the inability to select and compare accurately the cross section of the same anatomical area in images that have been obtained by imagers at different times or by images obtained essentially at the same time using different image modalities, e.g., CT and MRI. The inaccuracy in image comparison can be better appreciated from an explanation of the scanning techniques and how the imaging systems generate the images within a cross-sectional "slice" of the patient's anatomy. A slice depicts elemental volumes within the cross-section of the patient's anatomy that is exposed or excited by a radiation beam or a magnetic field and the information is recorded on a film or other tangible medium. Since the images are created from slices defined by the relative position of the patient with respect to the imager, a change of the orientation of the patient results in different elemental volumes being introduced into the slice. Thus, for comparison purposes, two sets of image slices of approximately the same anatomical mass taken at different times do not provide comparable information that can be accurately used to determine the changes that occurred between two image slices in the sets, since it is unknown to what extent the two individual image slices selected from the respective sets depict identical views.
The adverse effects on the medical practice of such errors is exemplified by diagnostic techniques utilized by the surgeon or others in diagnosing a tumor within a patient. If a patient has a tumor, its size density and location can be determined with the help of images generated by a scanning device. For the clinician to make an assessment of the patient's treatment, two scanning examinations are required. The patient is subjected to an initial scan that generates a number of slices through the portion of the anatomy, for instance the brain, to be diagnosed. During scanning, the patient is held in a substantially fixed position with respect to the imager. Each slice of a particular scan is taken at a predetermined distance from the previous slice and parallel thereto. Using the images of the slices, the doctor can evaluate the tumor. If, however, the doctor wants to assess changes in the configuration of the tumor over a given period of time, a second or "follow-up" scan has to be taken.
The scanning procedure is repeated, but since the patient may be in a position different from that in the original scan, comparison of the scans is hampered. Slices obtained at the follow-up examination may be inadvertently taken at an angle when compared to the original slices. Accordingly the image created may depict a larger volume than that which was actually depicted before. Consequently, the surgeon may get a false impression of the size of the tumor when comparing scans taken at different periods. Because of this, slice-by-slice comparison cannot be performed satisfactorily.
Similarly for certain surgical techniques it is desirable to have accurate and reliable periodic scans of identical segments of the tumor within the cranial cavity. If the scans before and after surgery are inaccurate, the doctor may not get the correct picture of the result of surgery. These same inaccuracies apply to other treatments such as chemotherapy discussed above.
Additionally, with regard to imaging systems and the integral part they play in surgical and other tumor treatment procedures, there is a dearth of methods currently existing that allow a determination of a desired location within the body at a given time. For example, U.S. Pat. No. 4,583,538 to Onik, et. al. discloses a localization device that is placed on a patient's skin which can be identified in a slice of a CT scan. A reference point is chosen from a position on the device which exactly correlates to a point on the CT scan. Measurements of the localization device on the CT scan is then correlated to the device on the patient.
Exterior devices have been utilized in an attempt to solve some of these problems with accuracy such as that shown in Perry U.S. Pat. No. 4,341,220 which discloses a frame that fits over the skull of a patient. The frame has three plates, each defining a plurality of slots on three of four sides. The slots are of varying lengths and are sequentially ordered with respect to length. Frame coordinates defined and found on the frame correspond to the varying heights of the slots. When slices of the skull and brain are taken by an imaging device, the plane formed by the slice intersects the three plates. The number of full slots in the slice are counted with respect to each plate to determine the coordinate of a target site with the brain. Accordingly, only one CT scan is needed to pinpoint the coordinates of the target.
Other attempts have included the use of catheters for insertion into the anatomy. For example, Codrington U.S. Pat. No 4,572,198 discloses a catheter with a coil winding in its tip to excite or weaken the magnetic field. The weak magnetic field is detectable by an NMR device thus pinpointing the location of the catheter tip with respect to the NMR device.
Applicant's invention largely overcomes many of the deficiencies noted above with regard to imagers used heretofore. The invention relates to a method and apparatus for insuring that scans taken at different times produce images substantially identical to those of previous scans even if they are from different image modalities at different times. This insures that a more accurate assessment of any changes in anatomy is obtained. As a result, the doctor can be more certain as to the size, location and density of the tumor, or a section thereof, that is located in the cranial cavity.
This ability will enhance the use of surgical techniques in removing or otherwise eliminating the tumor in particular by those noninvasive techniques such as laser technology. By having the ability to define accurately the tumor location and size, laser beams can be focused directly on the tumor. Intermittently, as part of surgical techniques, scans can be made to determine if the tumor has moved or substantially changed in size as a result of the surgery. The laser or other surgical instrument can be adjusted accordingly. Because of the accuracy of the imaging techniques produced by the invention, the doctor can be confident that the amount of healthy tissue destroyed during surgery is minimized.
A method adopted by the invention disclosed herein utilizes fiducial implants or implants to define a plane which cooperates with the imager, or other computer, and particularly the data processing capabilities of the imager to insure that subsequent scanning results in slices substantially parallel to those taken during the initial scan. The fiducial implants are implanted beneath the skin into the calvaria and are spaced sufficiently from one another to define a plane. The patient with these implants implanted is placed in the scanning device in the conventional manner and scanned to provide the images of consecutive parallel slices of a given thickness along a predetermined path through the cranial cavity.
As the scans are taken, one or more slices will be needed to accommodate part or all of each fiducial implant. The computational features of the imager or other computer will take into account the spatial relationship between any selected plane of a slice and that plane defined by the fiducial implants. Because of this capability, images taken in subsequent scans at different points in time, at different angles can be reconstructed to be substantially identical with the slices taken originally.
Fiducial implants for this purpose are specially configured and made of material that enables their implantation into the skull and the ability to be detected by scanning devices. The fiducial implant as disclosed herein is configured to insure that during implantation it does not have adverse effects on the skull such as cracking or extending through to the cranial cavity. Nor is it sufficiently exposed between the skull and the skin to distort any external features of the anatomy. Furthermore, the fiducial implant is positioned at least on a portion of the skull at the interface of the skin and the bone of the skull to facilitate its imaging by the imager. At least a portion of the implant is symmetrical in cross-section such that slices taken of the cranial cavity for example can be used to locate the center of mass of the implant. This insures accuracy in using the implant image as a reference point to transform the subsequent slices of the follow-up examination into the proper position and orientation.